Accelerometers are electromechanical devices that are widely used to measure acceleration forces due to motion and/or vibration. Capacitive accelerometers may find use in applications including seismic sensing, vibration sensing, inertial sensing and tilt sensing. Capacitive accelerometers are typically implemented as micro electromechanical systems (MEMS) and maybe manufactured from a semiconductor material such as silicon. A typical MEMS sensing structure for a capacitive accelerometer comprises a proof mass moveably mounted to a support, with a set of electrode fingers extending from the proof mass being interdigitated with one or more sets of fixed electrode fingers so as to form a differential capacitor. The electrodes of the sensing structure are connected to suitable drive and pickoff electronics. In an open loop configuration, the electronics are arranged to drive the fixed electrode fingers with sine or square wave signals and the proof mass moves under acceleration to provide a pickoff signal that is a rectified voltage appearing on the output. WO 2004/076340 provides an example of an open loop accelerometer. However, open loop accelerometers can have limited performance in terms of bandwidth, linearity and dynamic range.
An accelerometer sensing structure designed for open loop operation can also be used in a closed loop configuration by using drive electronics to provide a variable electrostatic force to the electrodes to achieve force balancing. WO 2005/084351 provides an example of a closed loop electronic control circuit using pulse width modulation (PWM) of the drive signals. In such a closed loop configuration, the electronics are arranged to drive pairs of the fixed electrode fingers in anti-phase using PWM signals so that the proof mass is fixed in position by virtue of the electrostatic forces nulling the inertial force due to acceleration. The mark:space ratio of the PWM drive signals can be adjusted to produce a variable rebalance force. Some conventional closed loop accelerometers have used separate transducers for the forcing and sensing functions, i.e. separation in the time domain. In another approach, a combined PWM forcing and sensing system enables the position of the proof mass to be determined simultaneously with applying the rebalance forces, by separating the excitation and feedback signals in the frequency domain. For example, for a proof mass having a resonant frequency of around 1-3 kHz, the PWM drive signals may be at a frequency of around 100 kHz. Feedback from the pickoff circuit to the PWM drive signal generator causes the length of each PWM pulse to be changed as a function of the pickoff output voltage so as to provide an average electrostatic restoring force maintaining the proof mass at a central null position. In such a closed loop implementation, the condition of equal mark:space ratio i.e. 50:50 corresponds to zero net output force and the null position of the proof mass.
Under high g acceleration, the mark or the space will grow in length, depending on the sense of the acceleration. The range of the mark:space ratio is theoretically available in the range 0% to 100%. However, in practice, the mark:space ratio must always be in the range of typically 10% to 90% to give sufficient settling time for the sensing system to accurately determine the position of the proof mass, which is essential for performance. Thus the pulse width (or duty cycle) cannot span the full 0% to 100% range because of the need to preserve sensing function. This then limits the dynamic range of the rebalancing electrostatic forces that can be applied, which in turn limits the dynamic range over which closed loop operation can be achieved. The practical 10% to 90% range of a PWM-based closed loop system is equivalent to the full scale range of a sensor where 10% is full scale negative, 90% is full scale positive and 50% represents a critical 0 g (bias) condition. The resolution and accuracy of the PWM system within the 10% to 90% range is then determined by the maximum available clock frequency and becomes a limiting factor in setting the overall measurement range and accuracy of the system.
Another problem with operating in an “over range” condition below the 10% threshold or above the 90% threshold is that the electrode fingers of the proof mass will no longer be held in the null position as they are deflected close to the outer fixed electrodes—when this gap becomes small it can generate a large electrostatic force, since the electrostatic force varies with the inverse square of the gap. The proof mass can then “latch up” and can only be released by switching off the electronics and resetting the accelerometer. This is a fundamental drawback of analogue, or PWM, closed loop accelerometer systems. Although mechanical “bump stops” are normally included in the sensing structure to limit the displacement of the proof mass from extreme over range conditions, such bump stops are subject to manufacturing tolerances and are difficult to match exactly to the available PWM mark:space ratio range. This means that a latch-up condition may exist where the electrostatic force generated by the maximum pulse width (and large gap) can be exceeded by the electrostatic force generated by the minimum pulse width (and small gap), resulting in a latch-up condition e.g. if the bump stop size is inadequate or if the maximum g range of the device is greatly exceeded.
Of course the effective range of a closed loop accelerometer could always be increased so that high g accelerations previously considered an “over range” condition are now within the actual range of the accelerometer, but this is always at the expense of the low g performance unless a higher resolution PWM system can be produced. The PWM range, resolution and accuracy is therefore a limiting condition which cannot easily be overcome conventional means. There will always exist a maximum g level set by the voltage available and the electrode gaps. Thus for any sensing structure there will be the potential for an over range condition with the risk of latch up, e.g. depending on the bump stop gaps.
Where a closed loop accelerometer is operated within its normal g range and the mark:space ratio does not go beyond the upper and lower thresholds of the 10% to 90% range, then none of these problems may be encountered. However, it is often not possible to control the g forces experienced by an accelerometer during use. In real life, an acceleration larger than the dynamic range of the sensor may easily arise, at least temporarily, in extreme environments. For example, this could happen due to a shock in acceleration, which is often generated for a short period of time in a vehicle due to impacts from stones or closing of doors etc. In more extreme environments such as a rocket launcher, the accelerometer must be able to survive a high g for a limited period of time without latching up.
It would be desirable for a closed loop capacitive accelerometer to provide fault tolerance without its dynamic range being over-designed. The present disclosure seeks to reduce or overcome the disadvantages outlined above.